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Reaction rates. Two beakers showing the change in the reaction rate of zinc with a strong (left) and weak (right) acid. A stronger acid is one that forms more hydrogen ions than a weak acid. These hydrogen ions react with the zinc (and most other metals). The greater amount of hydrogen ions in the same volume (higher concentration) means that more hydrogen ions are available at the surface of the zinc to react, hence the faster reaction rate. Metals react with acids to form hydrogen gas, seen as the bubbles in the test tubes, and a chemical salt. This is sulphuric acid so the salt formed is zinc sulphate, and stays dissolved in the water.